Methods for surface texturing used in the photovoltaic industry are often time consuming and inefficient. In order to obtain photovoltaic cells with high conversion efficiency, one side (e.g. the front side or front surface) of the cells is preferably textured while the other side (e.g. the rear side) is polished. The front side is textured to substantially reduce light reflection such that a substantial amount of the incident light is captured by the photovoltaic cell. The rear side is polished such that it can act as a mirror for reflecting light (e.g. infrared light) that passes through the substrate without being absorbed. The light reflected at the rear side can pass a second time through the substrate, thereby increasing the chances of being absorbed and creating charge carriers.
In industrial manufacturing processes for high efficiency silicon photovoltaic cells, the front silicon surface is generally textured by means of wet chemical etching. For example, alkaline based solutions such as KOH/IPA or NaOH/IPA (IPA: Isopropyl alcohol) based solutions are used for texturing monocrystalline silicon substrates. For multicrystalline silicon substrates HF/HNO3 mixtures are often used for texturing.
In a typical industrial process, starting from as-cut silicon wafers, first a saw damage removal (SDR) step is performed, typically removing a silicon layer of about 10 micrometer thickness at both sides of the wafers. Next, both sides are textured by wet etching, thereby removing typically about 5 micrometer of silicon at both sides of the wafers. Subsequently the rear side of the wafers is polished using a one-side polishing process, e.g. a ‘floating wafer’ process wherein only the rear side of the wafers is submerged in an etching solution. This results in an additional removal of typically 10 micrometer of silicon. Such a process sequence results in a reduction of the silicon wafer thickness with typically 40 micrometer. This means that for industrial silicon wafer thicknesses of 180 micrometer, the silicon loss is more that 20%. In addition to the silicon loss the wet processing is rather time consuming and hinders high throughput processing that is required for cost effective photovoltaic cell processing.
The silicon loss can be reduced by providing a masking layer, e.g. a dielectric layer, at the rear side of the substrate after saw damage removal and before the texturing step. Using such an approach, the reduction of the silicon wafer thickness can be limited to typically 25 micrometer. However, it requires an additional process step for providing the masking layer.
Another method that can be used for single side texturing is plasma texturing (dry etching). Although it is a one sided process that can be scaled up to increase throughput, a wet etching step is still required for polishing the rear side to ensure removal of saw damage.